Coordinate measuring instrument with feeler element and optic sensor for measuring the position of the feeler

ABSTRACT

A method for measuring structures of an object using a feeler element associated with a coordinate measuring instrument and extending from an elastic bendable feeler extension is disclosed, and wherein the feeler element is brought into contact with an object having structures to be measured and the position of the feeler is then determined by comparing the position of the feeler as determined by the coordinate measuring instrument with the position determined by the optical sensor.

CROSS-REFERENCE TO RELATED APPLICATIONS:

Not Applicable

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The invention relates to a method for measurement of an object by meansof a feeler element used with a coordinate measuring instrument andextending from a feeler extension, where the feeler element is broughtinto contact with the object and its position is then determined. Theinvention further relates to a coordinate measuring instrument formeasurement of structures of an object by means of a feeler used with acoordinate measuring instrument and comprising a feeler element and afeeler extension, a sensor for optical determination of the feelerelement and/or at least one target directly assigned thereto, and anevaluation unit using, wherein the structures can be calculated from theposition of the optical system relative to the coordinate system of thecoordinate measuring instrument and from the position of the feelerelement and/or of the target measured directly using the optical system.

2. Description of Related Art

For measurement of the structures of an object, coordinate measuringinstruments with electromechanically operating feelers are used withwhich the structure position is determined indirectly, i.e. the positionof the sensing element (ball) is transmitted via a feeler pin. Theattendant deformations of the feeler pin in conjunction with the activefriction forces lead to a falsification of the measurement results.Because of the strong force transmission, measurement forces also resultthat are typically in excess of 10 N. The geometric design of suchfeeler systems limits these to ball diameters greater than 0.3 mm.

The three-dimensional measurement of small structures in the range of afew tenths of a millimeter and the sensing of easily deformed testspecimens is therefore problematic, if not impossible. As a result ofthe not completely known error influences due to deformation by thefeeler pin and feeler element, and the unknown sensing forces due tostick-slip effects for example, measurement uncertainties occur that aretypically in excess of 1 μm.

It is known from WO 93/07443 to indirectly determine the structure of anobject by means of optical sensors, where a rigid feeler has at leastthree targets, which are measured for determination of a coordinationmeasurement point using an angle sensor.

Another possibility for optical measurement of the structures of a bodyis described in WO 88/07656 by an interferomter system. This systemcomprises a feeler with a rod-like feeler extension at the end of whicha ball is arranged that is brought into contact with the body whoseposition is to be determined. The feeler extension extends from aplate-like holder that is adjustable in three dimensions relative to theobject. Retroreflectors extend from the holders and are subjected tobeams emitted by interferometers. The reflected beams are then measuredby the interferometers in order to permit measurement of the opticalaxis between the interferometers and the retroreflectors for thedetermination of the position of the object.

The publication US-Z.: Quality, April 1998, p. 20 ff contains theproposal of measuring structures of an object by means of a feelerelement by determining its position with an optical sensor. Here it isimportant that the feeler is sharply imaged.

It is known from the publication US-Z.: American Machinist, April 1994,p. 29-32, to use various measuring systems for the determination of thegeometry of a workpiece. In this case it relates to the possibility, onethe one hand, of measuring a surface with a video camera, and on theother hand, of performing a tactile measurement, these being treated asalternatives.

In US-Z.: Tooling & Production, October 1990. p 76-78, a feeler is usedoptionally for purely tactile, i.e. mechanical measurement and foroptical measurement to determine structures. In this case, the feelercontacting the body must also be clearly optically imaged at all times.

A corresponding mechanical-feeler coordinate measuring instrument isshown for example in German Patent 43 27 250 A1. Here a visual check ofthe mechanically sensing process can be made with the aid of a monitorby observation of the feeler head using a video camera. This feeler headcan if necessary be designed in the form of a so-called oscillatingcrystal feeler that is cushioned upon contact with the workpiecesurface. The video camera therefore permits bracing and control on themonitor of the position of the feeler ball relative to the workpiece orto the hole therein which is being measured. The measurement proper isconducted electromechanically, so that the above drawbacks remain valid.

An optical observation of a feeler head in a coordinate measuringinstrument is also shown in German Patent 35 02 388 A1.

To determine the precise position of the machine axes of a coordinatemeasuring instrument, at least six sensors are attached on a sleeveand/or to a measuring head in accordance with German Patent 43 12 579A1, for enabling the distance from a reference surface to be determined.The sensing of the object geometries is not dealt with in detail here,instead a proximity-type process as a substitute for the classicincremental path measurement systems is described.

U.S. Pat. No. 4,972,597 describes a coordinate measuring instrument withone feeler, of which the feeler extension is pretensioned in itsposition by a spring. A feeler extension section passing inside thehousing has two light-emitting elements located at a distance from oneanother for determining by means of a sensor element the position of thefeeler extension, and hence indirectly that of a feeler element arrangedon the outer end of the feeler extension. The optical system here alsoreplaces the classic path measurement systems of electromechanicalfeeler systems. The sensing process proper is again achieved by forcetransmission from the feeler element to the feeler pin via springelements to the sensor. The aforementioned problems with bending andsensing force remain here too. This method is indirect.

To measure large objects such as aircraft components, feeler pins withlight sources or reflecting targets are known, the positions of whichare optically measured (German Patents 36 29 689 A1, 26 05 772 A1, and40 02 043 C2). The feelers themselves are moved manually or by usingrobotics along the surface of the body to be measured.

With this method, the position of the feeler element is stereoscopicallydetermined in its position by triangulation or similar means. Theresolution of the overall measurement system is hence directly limitedby the sensor resolution. The use of such systems is therefore possibleonly in the case of relatively low requirements as regards therelationship of measurement area and accuracy. In practice their use islimited to the measurement of large parts.

Aiming at the position of the feeler element using a microscope is alsoknown. In this case, the transmitted-light method is used, so that onlystructures such as all-through holes or similar can be measured inrespect of their diameters. In view of the visual evaluation in themicroscope and the separate arrangement of feeler element and opticalobservation system, neither measurement of more complex structures(distances in complex geometries, angles etc.) nor automatic measurementis possible. Systems of this type are as a result highly prone to faultsand are therefore not offered on the market.

SUMMARY OF THE INVENTION

The problem underlying the present invention is to develop a method anda coordinate measuring instrument of the type mentioned at the outsetsuch that any structures can be determined with a high degree ofmeasurement accuracy, with the aim of precisely determining the positionof the feeler element to be brought into contact with the object. Inparticular, it should be possible to measure out bores, holes, undercutsor similar, and to determine structures in the range between 50 and 100μm with a measuring accuracy of at least ±0.5 μm.

The publication US-Z.: Plastics World, August 1989, No. 8, includes anillustration of a feeler element whose position is optically measured.As this illustration makes clear, a feeler is used that does not permitmeasurement of very small dimensions or of materials that are very softand hence must not be subjected to high sensing forces, as otherwise afalsification of the geometry would result. An appropriate disclosure isalso made in US-Z.: Quality, January, 1990, as the illustration makesclear.

A bore measuring microscope is known from the publication of Carl ZeissJena, Technische Messgerate, p. 54 and 55. In this teaching only thedistance between two diametrically opposite points of a bore to bemeasured is determined under microscopic observation using thetransmitted light method.

The problem is solved in accordance with the invention substantially inthat the feeler element is connected via an elastic-to-bending shaft asthe feeler extension to the coordinate measuring instrument, in that theposition of the feeler element or of a target extending from theelastic-to-bending shaft and directly assigned to the feeler element isdirectly determined with an optical sensor. For measurement of thestructure of the object using the optical sensor, certain coordinates ofthe feeler element or of the target are linked with those of thecoordinate measuring instrument, with the position being determined inthe transmitted light or reflected light method and/or by self emissionof the feeler element or target. Here the feeler element and/or the atleast one target is moved from the area of the optical sensor into theposition to be measured. In other words, the feeler is moved towards theobject from its side facing towards the sensor. The feeler and sensorare here adjustable as a unit inside a coordinate measuring instrumentand their joint position can be measured with high precision. This isfollowed by a linked movement that ensures relatively low uncertainty inthe results. Here the position in particular of the feeler elementand/or of the at least one target is determined using the sensor bymeans of light beams reflecting from and/or penetrating the objectand/or emanating from the feeler element or target.

In accordance with the invention, the position of the feeler elementresulting from contact with the object is determined optically, in orderto measure the shape of a structure directly from the position of thefeeler element itself or of a target. Here the deflection of the feelerelement can be measured by displacement of the image on a sensor fieldof an electronic image-processing system using an electronic camera. Itis also possible to determine the deflection of the feeler element byevaluation of a contrast function of the image. A further possibilityfor ascertaining the deflection is to determine it from a size change ofthe target image, from which results the geometrical-optical correlationbetween object distance and enlargement. Also, the deflection of thefeeler element can be determined by the apparent target size changeresulting from the loss of contrast due to defocusing. As a generalprinciple, the deflection vertical to the optical axis of the electroniccamera is determined here. Alternatively, the position of the feelerelement or of the at least one target assigned thereto can be determinedby means of a photogrammetric system. If several targets are present,their position can be optically measured and then the position of thefeeler element computed, as there is a clear and firm correlationbetween this and the targets.

In accordance with the invention, and in a divergence from the previousprior art, indirect measurement of the position of the feeler element orof the target assigned thereto takes place in order to determine thestructure of an object. Here the feeler element and the target have aclear spatial correlation to the extent that a relative movement to oneanother does not take place, i.e. short spacings are maintained. It isimmaterial here whether the feeler extension from which the target orfeeler element extends is deformed during the measuring process, sincethe feeler element or the target is not indirectly measured, as in theprior art, but directly. With the method in accordance with theinvention, holes, bores, depressions, undercuts or other structures withan extent in the range of at least 50-100 μm can be measured with anaccuracy of at least ±0.5 μm. This enables three-dimensionalmeasurements of very small structures to be performed, a requirementwhich has long been felt for example in medical technology for minimallyinvasive surgery, in microsensor systems, or in automotive engineeringto the extent that injection nozzles, for example, are concerned, butwhich has not yet been satisfactorily solved. Thanks to the directmeasurement of the feeler element position or of the target clearlyassigned and not movable relative thereto, a direct mechanical/opticalmeasuring method using a coordinate measuring instrument is providedthat operates with high precision and does not lead to falsifications ofmeasuring results even if the feeler extension becomes deformed duringthe measuring process.

A coordinate measuring instrument of the type mentioned at the outset ischaracterized in that the feeler extension is designed to be elastic tobending. The feeler element and/or the at least one target can here bedesigned self-radiating and/or as a reflector.

The feeler element and/or the target should preferably be designed as abody such as a ball or cylinder spatially emitting or reflecting a beam.

In accordance with the invention, the feeler element is connected to afeeler extension such as a shaft that is designed to be elastic tobending. This connection can be made by gluing, welding or by any othersuitable type of fastening. The feeler element and/or the target canalso be a section of the feeler extension itself. In particular, thefeeler extension or the shaft is designed as or incorporates a lightguide via which the necessary light is supplied to the feeler element orto the target. The shaft itself can be designed as a feeler at its endor can incorporate a feeler. In particular, the feeler element and/orthe target should be interchangeably connected to the feeler extensionsuch as a shaft.

In order to determine almost any structure, it is furthermore providedthat the feeler extends from a holder adjustable in five degrees offreedom. The holder itself can in turn form a unit with the sensor or beconnected to the sensor.

It is also possible for the feeler element and/or the target to bedesigned as or to incorporate a self-lighting electronic element such asan LED.

In accordance with the invention, a feeler system for coordinatemeasuring instruments is proposed that combines the advantages ofoptical and mechanical feeler systems, and which can be used inparticular for the mechanical measurement of very small structures whereconventional feeler systems can no longer be employed. However, simpleattachment and changing of optical measuring instruments for mechanicalmeasuring tasks is also possible as a result.

For example, it is provided that a feeler element or sensing element ora target assigned thereto can be determined in its position by a sensorsuch as an electronic camera once the former has been brought intomechanical contact with a workpiece. Since the position of either thefeeler element itself or the target connected directly to the feelerelement is measured, deformations of a shaft receiving the feeler haveno effect on the measuring signal. In the measurement, it is notnecessary for the elastic behavior of the shaft to be taken intoconsideration, and plastic deformations, hystereses and drift effects ofthe mechanical connection between the feeler element and the sensorcannot impair measurement accuracy. Deflections in the directionvertical to the sensor axis such as the camera axis can be determineddirectly by displacement of the image in a sensor field in particular ofan electronic camera. The evaluation of the image can be performed withan image-processing system already installed in a coordinate measuringinstrument. This provides a two-dimensionally operating feeler systemwhich can be easily connected to an optical evaluation unit.

For sensing the deflection in the direction of the optical system axissuch as a camera axis, there are several possibilities in accordancewith the invention, for example:

1. The deflection of the feeler element in the direction of the sensoraxis (camera axis)is measured by a focus system as already known inoptical coordinate measurement technology for focusing on workpiecesurfaces. Here the contrast function of the image is evaluated in theelectronic camera.

2. The deflection of the feeler element in the direction of the sensoraxis or camera axis is measured by the imaging size of a target beingevaluated, e.g. in the case of a circular or annular target the changein the diameter. This effect is the result of the geometrical-opticalimaging and can be selectively optimized by the design of the opticalunit. In coordinate measurement technology, so-called telecentric lensesare frequently used and are intended to achieve a largely constantenlargement even in the event of deviation from the focal plane. This isachieved by moving the optical entry pupil into “infinity”. For theevaluation as described above, an optimization the other way round wouldbe useful: even a minor deviation from the focal plane should result ina clear change of the imaging scale. This is achieved by for examplemoving the optical entry pupil to the level of the focal point on theobject side. If possible a high depth of field should be achieved topermit high-contrast imaging of the target over a relatively widedistance range. An ideal optical unit as regards its imaging propertiesfor the application described above would be for example a pin camera.By the use of an annular target, size changes resulting from lack offocus can be minimized: it is not the mean ring diameter that changesdue to lack of focus in the first approach, but only the ring width.

3. In a third option too, the size change of the target is evaluated,however this change results from the combination of geometrical-opticalsize change and the apparent enlargement by out-of-focus edges. Incomparison with the evaluation of the lack of focus function, thismethod takes advantage of the fact that the actual size of the target isinvariable.

In accordance with the invention, direct measurement of a feeler elementposition is used for determining the structures of objects. Generallyspeaking, many different physical principles are usable for the directmeasurement. Since the measurement of the feeler element deflection in avery large measurement range in the spatial sense must be very precise,for example to permit continuous scanning operations, and to allow for alarge excess stroke during object sensing (e.g. for safety reasons, butalso to reduce the effort needed for precise positioning), aphotogrammetric method can also be used. Two camera systems with axesoblique to one another can be used. In general the evaluation techniquesknown from industrial photogrammetry can be used.

With two cameras “looking” for example obliquely toward the longitudinaldirection of the feeler element or to the ends facing said feelerelement of a feeler extension such as a shaft, all measuring tasks canbe performed in which the feeler element does not “disappear” behindundercuts. The use of a redundant number of cameras (e.g. three) permitsmeasurement of objects with steep contours too. For measurement in smallbores, a camera can be used that is arranged such that it is “looking”onto the feeler element in the longitudinal direction of the feelerelement or feeler extension. As a general principle, a single cameraaligned with the longitudinal direction of the feeler extension such asshaft holding the feeler element is sufficient in the case oftwo-dimensional measurements (e.g. for measurements in bores).

For the use of the feeler in accordance with the invention, an activelylight-emitting feeler element or other active target is not essential.Particularly high accuracies can be achieved with light-emitting feelerballs or other light-emitting targets on the feeler extension. The lightfrom one light source is here supplied to the feeler element such asball or to other targets of the feeler extension for example via a lightguide fiber which can itself be the feeler shaft or feeler extension.The light too can be generated inside the shaft or in the targets ifthese contain LEDs, for example. The reason for these designs is thatelectronic image systems such as photogrammetric systems, in particularthose for microscopically small structures, require a high lightintensity. If this light is directly supplied to the feeler element intargeted form, the necessary light intensity can be reducedconsiderably, and hence also the thermal load on the object during themeasurement. If a ball is used for the feeler element, the result is anideally high-contrast and ideally circular image of the feeler ball fromevery direction viewed. This applies in particular in the use of avolume-dispersing ball. Errors from imaging of structures of the objectitself are avoided, since the object itself is only brightly illuminatedin the immediate vicinity of the feeler ball. Here however the feelerball image resulting from reflections on the object in practice alwaysappears less bright than the feeler ball itself As a result, errors canbe corrected without difficulty. Externally illuminated targets do notnecessary have these advantages. It is also possible to design thetargets fluorescent, so that incoming and outgoing light is separated interms of frequency, and hence the targets too can be more clearlyisolated from their surroundings in the image. The same considerationsapply for the feeler element itself.

To measure in small bores or on very steep structures too, when thefeeler element cannot be measured itself or not measured by severalcameras due to shading, the position, the orientation and the curvaturesof the light guide fiber in the visible part-areas can be measured inaccordance with the invention by sensors or by photogrammetry. From thisthe position of the feeler element can be calculated, e.g. by applyingthe fiber curvature in the form of a parabola with linear or squareterm. The measurement with different excess strokes (more or lesspositioned into the object) and then taking the mean of the feelerelement position increases measuring precision. Both optical andphotogrametric measurement of the fiber is facilitated by a steady lightemission of the fiber, which can be improved by the use ofvolume-dispersing fiber material, the application of a diffuselyemitting layer on the fiber surface or another suitable selection offiber composition and fiber geometry (e.g. production using materialwith relatively low refractive index).

It is also possible in accordance with the invention to attach furtherilluminated balls or other targets on the light guide fiber, to measurethe position of these targets by photogrammetry in particular, and tocalculate the position of the feeler element accordingly. Balls are hererelatively speaking ideal and clear targets that are not otherwisepresent on the fiber. A good light incidence into the balls is achievedby disrupting the light guidance properties of the shaft, for example bymounting the volume-dispersing balls with through-holes onto the shaft,i.e. the feeler extension, and gluing them there. The volume-dispersingballs can also be affixed to the side of the shaft, which also permitslight incidence, provided that the shaft carries light up to itssurface, i.e. does not have a sheath at the fastening point. Aparticularly high accuracy is achieved when the feeler element positionis experimentally measured (calibrated) as a function of the fiberposition and fiber curvature (zones of fibers at some distance fromfeeler element). Here too the measurement of targets attached along thefiber is possible instead of measurement of the fiber itself

Calibration can for example be achieved by sensing a ball from differentdirections and with different forces (more or less “positioned into” theobject), or by a known relative positioning of the feeler systemrelative to the clamped feeler ball.

The separation of the feeler element, such as feeler ball, and targetsadditionally reduces the possibility of disruption of the feeler elementposition measurement by reflections of the targets on the objectsurface.

In accordance with the invention, several feelers can be in useconsecutively; for example, various feeler elements or feeler pins canbe rotated into view with a simple changing unit (e.g. turret withseveral feelers). It is also possible in accordance with the inventionfor several feeler elements to be in operation at the same time. Theactive feeler element or feeler pin can for example be identified byswitching off the lights of the non-active feeler pins or by othercoding means such as target size, light color, target position in feelercoordinate system, modulation of the light and/or using attached models.Feeler pin measurements as standard in classic coordinate measurementtechnology are no longer essential in the feelers in accordance with theinvention, since the feeler ball position and the feeler ball diametercan be measured with often sufficient precision by photogrammetricmeans.

Measurement with small feeler elements often entails a large number ofdestroyed feeler pins (feeler element, feeler extension). With thesystem in accordance with the invention, the feeler pins are inexpensiveand easy to replace. Expensive sensors and the movement axes aregenerally not damaged or altered by collisions, since the distance fromthe feeler element can be quite large. For example, the shaft length canbe greater than the movement range of the system, so a collision is notpossible. A large feeler or ball deflection relative to the shaft lengthis possible without difficulty. The result is a high inherent safety ofthe system and good scannability. Also, high sensing speeds are possiblewithout damaging the object surface.

Photogrammetric systems, or other known optically operating sensorsystems, permit mathematical alignment of the object before the actualstart of measurement thanks to the image information from the lens. Thispermits accurate sensing of the object in the actual tactilemeasurement.

There are in this system two types of elastic influences that can leadto measurement deviations:

1. The resilience of the object itself (in large ranges); influencesfrom this can be extrapolated to zero by measurement with at least twosensing forces; and,

2. The local resilience from Hertzian stress between ball and objectsurface; these effects can if required (i.e. for high-precisionmeasurement or for resilient objects) be eliminated by a measurementwith at least two different sensing forces and extrapolation to thefictitious sensing force “zero”.

The extrapolation to the force “zero” in the second ease is possiblesince the deformation according to Hertz is equal to a constantmultiplied with the (sensing force)^(⅔)

 D=K×F ^(⅔)

where:

D is deformation at the point of contact between object and feeler ball;

F is force (or a quantity proportional to the sensing force); and

K is constant.

D₁=K×F₁ ^(⅔)

D₂=K×F₂ ^(⅔)

D₁−D₂=K×(F₁ ^(⅔)−F₂ ^(⅔))

From the above is derived the value of K when the difference (D₁−D₂) isknown from the measurement and when F₁ and F₂ are known. It is nowpossible to calculate the deformations D₁−D₂ in relation to sensing with“zero” force. The force-proportional values are for example the movementdistances calculated starting from the first object contact.

Alternatively, these can also be measured with force sensors. A forcesensor for example can be the fiber itself if its curvature isphotogrammetrically measured or on the basis of changes in the lightreflected/diffused back internally to the light source or in the emittedlight. It is best to perform the measurement with several sensing forcesfor all high-precision measuring tasks, since the effective radii in thecontact point between the object and the feeler element can vary greatlydue to local waviness and roughness features.

If the Hertzian and the linear resilience are of the same order ofmagnitude, sensing with at least three forces is necessary, and both thelinear and the Hertzian resilience constant must be determined in orderto extrapolate to the fictitious “zero” force.

If the divergences from the ideal spherical form in small balls used asfeelers are not negligible, with diameters of less than 0.1 m, adirection-dependent correction of the sensing point coordinates may benecessary. To measure the correction values, two methods are possible:

1. measurement of the deviations of the feeler element from thespherical form, performed independently of the feeler system withspecial measuring instruments; and

2. measurement of the deviations of the feeler element from thespherical form, performed by measurement of a reference ball with thefeeler system itself.

As a general principle, it is also possible to select a differentgeometry form for the feeler elements than that of a ball, e.g. acylinder, which can represent the fiber itself or the rounded end of thefiber itself as the feeler extension.

Since the feeler element (e.g. a ball) is more or less completely imageddepending on the direction of observation, and since dirt too has a verydisruptive effect, it is best to determine the position of the feelerelement with so-called robust compensation algorithms. These include forexample the minimization of the sum of deviation amounts (so-called L1standard).

Correction methods set forth above are however only necessary in extremecases, without the teachings in accordance with the invention beinggenerally affected as a result.

Generally speaking, the illumination of the feeler element, the targetsor the shaft can be not only from the inside through the shaft, but alsofrom the outside using suitable illumination devices.

One variant that is possible here is for the feeler element or targetsto be retro-reflectors (triple reflectors, cat's eyes, reflecting balls)that are externally illuminated from the camera viewing angle.

The feeler in accordance with the invention is generally not itselfrestricted to certain sizes of the measurement objects and feelerelement itself. It can be used for measurement of single-dimensional,two-dimensional or three-dimensional structures. In particular thefeeler extension can be designed as a light guide and have a diameter of20 μm. The diameter of the feeler element such as feeler ball shouldthen be preferably 50 μm. In particular, it is provided that thediameter of the feeler element is about 1 to 3 times greater than thatof the feeler extension.

To increase the fracture strength of the feeler extension when lightguides are used, the latter can have a surface coating such as Teflon oranother fracture-inhibiting substance. Sheathing can be applied bysputtering, for example.

The spatial position of the feeler element can be determined using atwo-dimensional measuring system when the feeler element has at leastthree targets assigned to it, the images of which are evaluated fordetermining the spatial position of the feeler element.

The invention also permits a scanning method for determining workpiecegeometries. In particular, the images to be evaluated can be generatedby a position-sensitive surface sensor.

Compared with purely mechanically measuring feeler systems, theteachings in accordance with the invention have the followingadvantages, among others:

Elastic and plastic influences and creepage effects of the mechanicalholder and the sensing shaft do not affect the measuring result.

Very low sensing forces (>1 N) can be attained.

No precision mechanics are necessary.

Very small feeler elements and shaft diameters can be used.

The positioning of the feeler system can be optimally monitored by theoperator using the optical system.

The systems can be directly attached to the existing optical system of acoordinate measuring instrument and the image signal evaluated using anexisting image processor.

Low equipment expenditure thanks to adaptation to existing opticalcoordinate measuring instruments.

Compared with purely optically measuring feeler systems, the advantagesare as follows:

The actual mechanical quantities are measured. Surface properties suchas color and reflection characteristics do not affect the measurementresult;

Measurements can be made on three-dimensional structures not accessiblefor purely optical systems. For example, the diameter and the formdivergence of a bore can be measured at different height sections.

BRIEF DESCRIPTION OF THE DRAWINGS

Further details, advantages and features of the invention are not onlyshown in the claims and in the features they contain, singly and/or incombination, but also in the following description of preferredembodiments shown in the drawings.

IN THE DRAWINGS,

FIG. 1 shows an embodiment of an array for measurement of structures ofan object.

FIG. 2 shows a second embodiment of an array for measurement ofstructures of an object.

FIG. 3 shows a third embodiment of an array for measurement ofstructures of an object.

FIG. 4 shows a fourth embodiment of an array for measurement ofstructures of an object.

FIG. 5 shows a fifth embodiment of an array for measurement ofstructures of an object.

FIG. 6 shows a sixth embodiment of an array for measurement ofstructures of an object.

FIG. 7 shows a section through a first embodiment of a feeler.

FIG. 8 shows a section through a second embodiment of a feeler.

FIG. 9 shows a section through a third embodiment of a feeler.

FIG. 10 shows a seventh embodiment of an array for measurement ofstructures of an object.

FIG. 11 shows an eighth embodiment of an array for measurement ofstructures of an object, and

FIG. 12 shows a block diagram.

PREFERRED EMBODIMENTS OF THE INVENTION

In the figures, in which identical elements are referred to by the samereference characters, various embodiments of arrays for the measurementof structures of an object are shown in principle by means of a feelerassigned to a coordinate measuring instrument. As an embodiment, thestructure of a bore 10 in an object 12 is to be determined. The edge ofthe bore 10 is sensed by a feeler element 14 which in turn extends froma feeler extension 16 and forms with the latter a feeler 18.

The feeler 18 extends from a holder 20 that is adjustable by at leastthree degrees of freedom, preferably five. An optical system of acoordinate measuring instrument 22 is preferably mounted on the holder20 itself. A different type of connection is also possible. It ishowever crucial that the optical system or a sensor of the coordinatemeasuring instrument 22 is adjustable as a unit with the feeler element14 in the X, Y and Z directions. Regardless of this, an adjustment ofthe feeler element 14 relative to the optical axis 24 and to the focalplane takes place. There are various possibilities for positioning thefeeler element 14, i.e. in one embodiment a feeler ball in theintersection of the optical axis 24 with the focal plane. It istherefore possible in accordance with the embodiment in FIG. 1 for thefeeler extension 16 to be inserted laterally from the holder 20 into theoptical axis 24.

In the embodiment in FIG. 2, fastening arms 26, 28 extend from theholder 20, end outside the focal plane and are used as a receptacle fora feeler extension 16 inserted laterally into the optical axis 24, saidfeeler extension being connectable by a coupling piece 30 to the feelerelement 32 which, via a rod-like section 34 passing along the opticalaxis 24, merges into the feeler element 14 proper in the form of a ball,using which the structure of the edge of the hole 10 is determined.

In the array according to FIG. 3, an L-shaped curved feeler extension 38is held by a receptacle 36 extending from the holder 20, with astraight-lined end section 40 of the feeler extension 38 runningparallel to the optical axis 24 and merging at the end into the feelerelement such as feeler ball 14.

Once the feeler element 14 has been adjusted, it can be observed throughthe existing optical system of the coordinate measuring instrument 22 oran appropriate sensor. When the edge of the bore 10 is sensed, thefeeler element 14 changes its position in the camera or sensor field.This deflection is evaluated by an electronic image-processing system.This achieves a mode of operation with a similar effect to aconventionally measuring feeler system. The coordinate measuringinstrument 22 can here be controlled in the same way as a conventionalmechanically measuring feeler system.

There are various possibilities for optical measuring of the feelerelement 14, and these are shown in principle in FIGS. 4 to 6 and 10 and11.

In FIG. 4, for example, a transmitted-light method is proposed, wherethe shadow of the feeler element 14 on the sensor or camera field isviewed or measured. It is essential for the transmitted-light method asshown in FIG. 4, however, that the workpiece 12 is passed throughcompletely.

In the embodiment in FIG. 5, the feeler element 14 is subjected to lightby reflecting in the light along the optical axis 24. To that end, thereis a mirror 42 above the coordinate measuring instrument 22 via whichlight is reflected in through the coordinate measuring instrument 22 andthe holder 20 along the optical axis 24.

A light guide fiber is preferably used for the feeler extension 38. Thishas the advantage that the light is passed through it to the feelerelement 14, as shown in FIG. 6. The light source itself is numbered 44in the Figure.

The feeler element 14 has in the embodiments a volume-emitting ballform, The feeler element 14 here can be firmly connected to the feelerextension 38 for example by gluing or welding. However aninterchangeable connection using a coupling is also possible.

While in the embodiment in FIG. 7 the feeler element 14 is glued to theend 40 of the feeler extension 38, in the embodiment in FIG. 8 thefeeler element 14, i.e. its end section 40, is itself designed as thefeeler element. To that end, the end section 40 is appropriately shapedat its end. It is however also possible to provide the end face of thefeeler extension 38 with a reflecting cover in order to fulfill thefunction of the target.

Instead of observation of the feeler element 14 itself, a preferablyspherical target 46 can be assigned to it in a fixed location, and is asection of the feeler extension 38 or is mounted thereon, as made clearin FIG. 11. The feeler extension 38 therefore has at its end thespherical feeler element 14. Furthermore, spherical targets 46, 48, 50are provided at intervals from one another on the feeler extension 38.As a result, it is possible to observe either the position of the feelerelement 14 directly or the targets 46 or 46, 48 or 46, 48, 50 clearlyassigned to it.

The feeler element 14 or the target 46, 48, 50 can be formed of variousmaterials such as ceramics, ruby or glass. In addition, the opticalquality of the appropriate elements can be improved by the applicationof coatings of diffusing or reflecting layers.

The diameter of the feeler extension 38 is preferably less than 100 μm,and preferably 20 μm. The feeler element 14 or the target 46, 48, 50 hasa greater diameter, preferably one between 1.5 and 3 times larger thanthat of the feeler extension 38 such as the light guide.

In the area where the sheath of the feeler extension 38 does not have tobe traversed by light, a surface coating of Teflon or anotherfracture-inhibiting substance can be provided. The image of the feelerelement 14 or of a target 46, 48, 50 assigned thereto can be displayedon, for example, a CCD field of an optical coordinate measuring machine.The displacement of the light dot in the CCD field can be measured withsubpixel precision. With the method in accordance with the invention,reproducible measurements with a precision in the μm range are possible.

In the embodiment in FIGS. 10 and 11, a photogrammetric method is used:two optical imaging systems such as cameras 52, 54 aligned with thefeeler element 14 extend from a common holder 20. The cameras 52, 54optically aligned with the feeler element 14 permit a spatialdetermination of the feeler element 14 using conventional evaluationtechniques known from industrial photogrammetry. The use of a redundantnumber of cameras (for example three) also permits measurement of anobject when one of the three cameras is shaded. For small bores, the useof one camera is sufficient, and in this way is optically aligned on thefeeler element 14. Independently of this, either an activelylight-emitting, light-reflecting or light-shading feeler element 14 or atarget 46, 48, 50 is used to determine the structure in the object, withlight being supplied from the light source 44 to the feeler element 14or to the targets 46, 48, 50 via the feeler extension 38 designed aslight guide fiber. Alternatively, it is possible to generate the lightitself in the feeler extension 38, or in the targets 46, 48, 50 orfeeler element 14 by these being or containing electrically illuminatedmodules such as LEDs, for example. With the teachings in accordance withthe invention, an ideally high-contrast image and an ideally circularimage of the feeler element 14 or of the targets 46, 48, 50 areobtained, provided the latter are of spherical form. Additionally oralternatively, it is possible to design the feeler element 14 or thetargets 46, 48, 50 fluorescent, so that incoming and outgoing light areseparated in frequency such that the image generated by the feelerelement 14 or the targets 46, 48, 50 can be separated from itssurroundings.

With the design in accordance with the invention of a coordinatemeasuring instrument using which a feeler element such as a feeler ballor a target clearly assigned thereto spatially is directly measuredoptically, in order to determine the structure of the body from thisdirect optical measurement of the feeler element or target, structuresin the order of magnitude of 100 μm and less, in particular in the rangeup to 50 m, can be determined with a measurement uncertainty of ±0.5 μm.With the coordinate measuring instrument, standard measurement volumesof, for example, 0.5×0.5×0.5 m³ can be measured.

FIG. 12 shows a block diagram in order to determine, in line with theteachings in accordance with the invention, the structure of an objectin a coordinate measuring instrument 56 by direct optical measurement ofthe position of a feeler element 58, where the object is to be sensed bythe feeler element 58 using CNC control.

The coordinate measuring instrument 56 is of standard design. Forexample the feeler element 58 extends from a holder attached to a sleeve62 adjustable in the X direction along a crosspiece 60, which in turn isadjustable in the Z direction. The object itself is fastened to ameasurement table 64 movable in the Y direction. When the feeler element58 senses the object, the coordinates X¹, y¹ and Z¹ of the feelerelement 58 are calculated from the video signals corresponding to theposition of the feeler element 58 by an image-processor 66, and then fedto a measurement computer 68 and there linked to the coordinate valuesX, Y, Z of the coordinate measuring instrument 56, which are determinedusing a counter 70. From the values computed in this way, on the onehand the object geometry is determined and on the other hand the CNCoperation of the coordinate measuring instrument 56 is controlled usinga CNC control 72.

What is claimed is:
 1. A method for measuring the structure of an object by means of a flexible feeler means that is operatively connected to a coordinate measuring instrument, wherein the feeler means is brought into contact with the object and the position of the feeler means is then determined, the feeler means comprising a mechanically flexible and deflectable feeler extension, wherein said feeler extension is deflectable due to its inherent flexibility, the position of the feeler extension, or of a target extending from the feeler extension and operatively connected to the feeler extension is directly determined with an optical sensor, and wherein certain coordinates of the feeler extension or of the target are linked with those of the coordinate measuring instrument for measuring the structure of the object, using the optical sensor. 2.Method according to claim 1, wherein the feeler extension or the target is moved towards the object from its side facing towards the sensor.
 3. Method according to claim 1, wherein the feeler extension is adjusted with the sensor as a unit.
 4. Method according to claim 1, wherein a deflection of the feeler extension resulting from the contact with the object is optically determined.
 5. Method according to claim 4, wherein the deflection of the feeler extension is measured by displacement of its image or of an image of a target on a sensor field.
 6. Method according to claim 4, wherein the deflection of the feeler extension is determined by evaluating the contrast of the feeler sensed by the optical sensor.
 7. Method according to claim 4, wherein the deflection of the feeler extension is determined from a size change of an image of a target resulting from the geometrical-optical correlation between object distance and enlargement.
 8. Method according to claim 4, wherein the deflection of the feeler extension is determined from apparent size change of a target resulting from loss of contrast due to defocusing.
 9. Method according to claim 4, wherein the deflection vertical to an optical axis of an electronic image-processing system is determined.
 10. Method according to claim 1, wherein the spatial position of the feeler extension is determined using a two-dimensional measurement system by means of at least three targets assigned thereto.
 11. Method according to claim 1, wherein the feeler extension or a section thereof is used as a spatially extended target whose position is measured relative to the feeler body in freely selectable cross-sections.
 12. Method according to claim 1, wherein targets arranged on the feeler extension for determining the position of the feeler means are measured by photogrammetry (at least two cameras).
 13. Method according to claim 1, wherein the position of the feeler extension is measured by photogrammetry (at least two cameras).
 14. A coordinate measuring machine for measuring the structure of an object, the machine comprising a support table arranged on the X-Y axes of the measuring machine for supporting the object, a holding device adjustable in the X-Y-Z axes of the measuring machine, feeler means including a mechanically flexible and deflectable feeler extension, wherein said feeler extension is deflectable due to its inherent flexibility, said feeler extension having a contact tip at its end and being operatively connected to said holding device, an optical sensor for determining the location of said contact tip or a target directly associated with said contact tip, and a measurement computer for calculating the structure of the object based on the position of the contact tip or the target.
 15. Coordinate measuring instrument according to claim 14 wherein the contact tip (14) or the target (46, 48, 50) is designed as a reflector.
 16. Coordinate measuring instrument according to claim 14, wherein the contact tip (14) or the target (46, 48, 50) is designed self-emitting.
 17. Coordinate measuring instrument according to claim 14, wherein the contact tip (14) or the target (46, 48, 50) is a ball or cylinder spatially emitting or reflecting a beam.
 18. Coordinate measuring instrument according to claim 14, wherein the contact tip extension (38) is designed at least in some sections elastic to bending or as a light guide or comprising a light guide.
 19. Coordinate measuring instrument according to claim 14, wherein the feeler extension (38, 40) or at least a section thereof is the feeler element (14) or the target (46, 48).
 20. Coordinate measuring instrument according to claim 14, wherein several targets (46, 48) are assigned to the contact tip (14) and extend preferably from the feeler extension (30) or form sections thereof.
 21. Coordinate measuring instrument according to claim 14, wherein the feeler extension (30) is designed L-shaped for alignment with an optical axis (24).
 22. Coordinate measuring instrument according to claim 14, wherein the feeler extension (30) is designed at the end as a feeler element (14).
 23. Coordinate measuring instrument according to claim 14, wherein the contact tip (14) or the target (46, 48, 50) are interchangeably connected to the feeler extension (30).
 24. Coordinate measuring instrument according to claim 14, wherein the contact tip (14) or the target (46, 48, 50) are connected to the feeler extension (30) by gluing or welding.
 25. Coordinate measuring instrument according to claim 14, wherein the feeler extension (18) extends from a holder (20) that is adjustable by at least three degrees of freedom, preferably five, and preferably interchangeable.
 26. Coordinate measuring instrument according to claim 14, wherein the feeler extension (18) extends from a holder (20) that forms a unit with the sensor or is connected to the sensor.
 27. Coordinate measuring instrument according to claim 14, wherein the feeler extension (18) is moved towards the object from its side facing towards the sensor.
 28. Coordinate measuring instrument according to claim 14, wherein the contract tip (14) or the target (46, 48, 50) has or is a self-lighting electronic element.
 29. Coordinate measuring instrument according to claim 14, wherein the sensor is an image-processing sensor.
 30. Coordinate measuring instrument according to claim 14, wherein the sensor is a position-sensitive surface sensor.
 31. Coordinate measuring instrument according to claim 14, wherein the diameter of the contact tip (14) is about 1 to 3 times greater than that of the feeler extension (38).
 32. Coordinate measuring instrument according to claim 14, wherein the feeler extension (30) has at its end a cylindrical form and is designed as a feeler element (14).
 33. Coordinate measuring instrument according to claim 14, wherein the feeler extension (30) is spherically rounded for formation of the feeler element. 